Update multiple rows with different values in a single query - MySQL

I'm new to MySQL.

I'm using this to update multiple rows with different values, in a single query:

UPDATE categories
    SET order = CASE id
        WHEN 1 THEN 3
        WHEN 2 THEN 4
        WHEN 3 THEN 5
    END,
    title = CASE id
        WHEN 1 THEN 'New Title 1'
        WHEN 2 THEN 'New Title 2'
        WHEN 3 THEN 'New Title 3'
    END
WHERE id IN (1,2,3)

I am using "WHERE" to improve performance (without it every row in the table would be tested).

But what if I have this senario (when I don't want to update title for id 2 and 3):

UPDATE categories
    SET order = CASE id
        WHEN 1 THEN 3
        WHEN 2 THEN 4
        WHEN 3 THEN 5
    END,
    title = CASE id
        WHEN 1 THEN 'New Title 1'
    END
WHERE id IN (1,2,3)

The above code will change the title for id 2 and 3 into "NULL"...

What is the right way to make the query, but skip updating title for id 2 and 3 and still keep the performance "WHERE id IN" gives ?

Maybe like this

UPDATE categories
    SET order = CASE id
        WHEN 1 THEN 3
        WHEN 2 THEN 4
        WHEN 3 THEN 5
    END,
    title = CASE id
        WHE开发者_运维知识库N 1 THEN 'New Title 1'
        WHEN  THEN 
        WHEN  THEN 
    END
WHERE id IN (1,2,3)

---

Set title equal to itself when you don't want to update it to a different value.

UPDATE categories
    SET order = CASE id
        WHEN 1 THEN 3
        WHEN 2 THEN 4
        WHEN 3 THEN 5
    END,
    title = CASE id
        WHEN 1 THEN 'New Title 1'
        ELSE title
    END
WHERE id IN (1,2,3)

---

UPDATE `table_name` SET `field_name1` = CASE `id`
WHEN '1' THEN 'value_1'
WHEN '2' THEN 'value_2'
WHEN '3' THEN 'value_3'
ELSE `field_name1`
END,
`field_name2`= CASE id
WHEN '1' THEN 'value_1'
WHEN '2' THEN 'value_2'
WHEN '3' THEN 'value_3'
ELSE `field_name2`
END